The man command is collectively referred to as the linux manual. It provides documentation for commands. Almost every command has a man "page". Pages are grouped into "chapters".

Syntax:

 man [<chapter>] [commands]

Example:

 man cal

Output:

CAL(1)          General Commands Manual         CAL(1)

NAME
     cal, ncal – displays a calendar and the date of
     Easter

SYNOPSIS
     cal [-3hjy] [-A number] [-B number] [[month]
         year]
     cal [-3hj] [-A number] [-B number] -m month
         [year]
     ncal [-3hjJpwy] [-A number] [-B number]
          [-s country_code] [[month] year]
     ncal [-3hJeo] [-A number] [-B number] [year]
     ncal [-CN] [-H yyyy-mm-dd] [-d yyyy-mm]

DESCRIPTION
     The cal utility displays a simple calendar in
     traditional format and ncal offers an alternative
     layout, more options and the date of Easter.  The
     new format is a little cramped but it makes a
     year fit on a 25x80 terminal.  If arguments are
     not specified, the current month is displayed.

     The options are as follows:

     -h      Turns off highlighting of today.

     -J      Display Julian Calendar, if combined with
             the -e option, display date of Easter
             according to the Julian Calendar.

     -e      Display date of Easter (for western
             churches).

     -j      Display Julian days (days one-based,
             numbered from January 1).